This is likely a trial balloon for something that will eventually be implemented on the rest of the country:

Seoul City will ban high-emission cars from the roads in the greater metro area from December to March as part of efforts to reduce fine dust during the winter months.
The city government announced on Sunday that anyone caught driving vehicles classified as Grade 5 in terms of emission levels inside the capital region will be fined 100-thousand won from December.
People can face fines if they drive Grade 5 vehicles without an emission-reduction device from 6 a.m. to 9 p.m. on weekdays in the capital region.
As of September, there are about one-point-46 million vehicles nationwide that fall into this grade.
KBS World Radio
I doubt that this will do much of anything in regards to the pollution when the majority of it is coming from China. This seems like the Korean government is saying they are “doing something” when they know the problem is unsolvable as long as China doesn’t change its own pollution policies.

